Download effects of roads on wildlife in an intensively modified

EFFECTS OF ROADS ON WILDLIFE IN AN INTENSIVELY
MODIFIED LANDSCAPE.
J. E. Underhill and P. G. Angold
School of Geography and Environmental Sciences, University of Birmingham, Edgbaston,
Birmingham B15 2TT.
Correspondence author: P. G. Angold, School of Geography and Environmental Sciences,
University of Birmingham, Edgbaston, Birmingham B15 2TT.
Pre-print of article published in
Environmental Review vol 8, 2000, pp 21-39
Publisher: National Research Council Canada
doi:10.1139/ER-8-1-21
1
Abstract
This paper examines the ecological impacts arising from road networks and the potential
ameliorating effects of roadside habitat in a highly modified landscape. A UK focus has been
adopted to illustrate the effects of roads in a landscape with a long history of land use and
intensive land management where the impacts and the potential for improvement are
considerable.
The impacts of roads in the ecological landscape include habitat loss, fragmentation, and
degradation. These interrupt and modify natural processes altering community structures and
in the longer term, population dynamics. The large number of fauna fatalities each year from
road traffic accidents is also of concern. Road verges can however also provide habitat
opportunities and restore connectivity in an otherwise fragmented landscape offering potential
to offset some of the adverse impacts of the existing road network. This review demonstrates
that roads can present both ecological costs and ecological benefits although currently there is
insufficient evidence to confirm some of the key theories which relate to the impact of the
barrier effects (at population level) or the value of road verges as ecological corridors. In the
absence of complete information the full extent of the problems and opportunities cannot be
gauged and every effort should be made therefore to enhance the habitat adjacent to existing
roads and to constrain further fragmentation caused by the development of the existing road
network. Where further construction is unavoidable conditions should be enforced to prevent
roads from reducing further the remaining habitats of conservation value and the connectivity
between such habitats.
Keywords roads, wildlife, fragmentation, corridor, barrier, environmental impact.
2
Introduction
The environmental impact of roads is of increasing international interest and concern
(Spellerberg 1998). The impacts of roads include habitat loss, habitat fragmentation and
habitat degradation which affect wildlife and its habitats both directly and indirectly (Table
1). Much of the debate on the effects of roads on wildlife has focussed on the barrier effect of
roads for the larger mammals which have big ranges or undertake seasonal movements over
large areas of mainly natural or semi-natural habitat (e.g. Gunther and Biel 1999, Paquet and
Callaghan 1996, Andrews, 1990). Less attention has been paid to the impacts of roads as
potential barriers or corridors for smaller animals and plants in a more modified landscape
with a long history of intensive land use and land management. In the UK, there are around
370,000 km of roadways which pervade the length and breadth of the British Isles. Only in
places such as central and northern Scotland, the northern-most parts of the UK, are there any
large continuous areas of semi-natural habitat which remain intact; traffic is audible from
virtually every location in England (DETR 1998). To illustrate the ecological impacts of
roads in an intensified landscape a case study is presented which demonstrates the potential
impact of roads on areas of nature conservation.
On the positive side, the road verge can function as a ‘green estate’ of considerable length.
The provision of linear vegetated verge may provide habitat for many species, a feature of
particular importance in a landscape with diminishing areas of undisturbed or (semi-)natural
habitat. In such landscapes the continuous nature of the road verge may also be important as
a connecting route for wildlife between remnant habitat patches.
This paper is concerned with the impact of the road and its verge in an intensified landscape,
3
and the influence it exerts on plants and animals locally. It considers only the major and
immediate impacts of roads and does not therefore consider secondary or remote effects such
as any stimulus provided for future development etc. in the neighbourhood of the road.
Case study: an analysis of the potential impacts of roads on areas of nature
conservation importance in Oxfordshire, England.
A map-based study has been undertaken for the county of Oxfordshire in England to illustrate
the potential impact of roads on areas of nature conservation importance in an intensified
landscape. All designated Sites of Special Scientific Interest (SSSI) in the county have been
assessed to determine site area, the extent of penetration by roads and the length of road
within the site. In the UK, SSSIs are selected to protect a high-quality, representative stock of
the country’s biological, geological and geomorphological resource.
This case study
therefore provides a good illustration of the degree to which roads affect areas of nature
conservation interest throughout the county.
There are 127 designated SSSIs in Oxfordshire, covering a total of approximately 4,150 ha
and ranging in size from less than 1ha to almost 500 ha. Seventy-two (57%) of these sites
have no road in or adjacent to them (Table 2). Forty-one sites (32%) have a road running
along side them but not within, and 14 sites (11%) have at least one road within the SSSI
boundary. Thus almost half of the designated SSSIs in Oxfordshire are affected by roads
within or adjacent to them. In the recent past 20% of new routes have passed through areas of
nature conservation value (English Nature 1993) although it is intended that SSSIs are
avoided where practicable, and full consultation is required when there is potential for either
direct or indirect damage or when a road is planned within 100m of an SSSI. In this study
roads were found in SSSIs of all sizes but a greater proportion of larger sites were affected.
In the largest site in this study there were four roads within the boundaries of the SSSI,
4
including the M40 motorway.
The impact of habitat loss and disturbance due to the road can be easily modelled. Assuming
a minor road of 5m width, the area lost can be calculated using the measured length of road
within the SSSI, and compared to the total area of the SSSI. The literature commonly cites a
disturbed or polluted zone up to 5m from the road, and a detectable impact on ecological
communities has been shown up to 100m from a road in the UK (Angold 1997a, 1997b).
Further calculations were therefore made of the area affected by the road assuming a zone of
impact 5m and 100m from the road into the habitat either side (Table 2).
A surface area equal to at least 1% of the total SSSI habitat and as much 10% of the small
sites was found to be occupied by roads. In the affected sites the proportion of habitat lost
increased with decreasing size of protected area. Taking a minimum 5m zone of impact from
the road, 2% of the total area of SSSIs but 30% of the small sites are affected. With a 100m
zone of impact only the larger sites retain any undisturbed and almost a quarter of all the
SSSIs protected landscape is disturbed by roads. This example demonstrates most effectively
the severity of the environmental impact of the road network in a highly fragmented
landscape.
The overall habitat loss beneath the road surface is itself significant when
considered in the context of the ecological importance of these sites (they are irreplaceable)
and in the context of the limited remaining area of conservation importance in the UK. The
area subject to disturbance and alteration illustrates the fragility of these remant sites in an
intensively modified and highly fragmented landscape, and the paucity of any unaffected
‘core’ area in all but the very largest sites.
5
Roads and verges from an ecological perspective
An obvious and pervasive effect of roads is the fragmentation of previously continuous
habitat. The effects of habitat fragmentation are well documented and include a direct loss of
habitat, an increased ratio of edge to habitat, a reduction in patch size and the isolation of
remnant habitat (Andren 1994, Spellerberg 1998 see also Canters and Cuperus 1997, Evink
et al 1998, Evink et al 1999). Where roads are the fragmenting feature there are additional
effects which include the impacts of pollutants, noise, mortality and the barrier effect of an
inhospitable linear terrain of indeterminate length. (Angold 1997a, Angold 1997b, Bennett
1991a and 1991b, Evink et al 1996, Reijnen and Foppen 1997, Slater 1995, Spellerberg
1998).
Roads as barriers
When habitats and their associated populations are fragmented into smaller units and the
normal interchange between individual species are severed, their long term persistence may
be threatened. Small and isolated populations are vulnerable to extinction in heterogeneous
landscapes because of inbreeding depression or as a result of stochastic events. Subsequent
re-colonisation is a frequent and a widespread phenomenon however (Fahrig and Merriam
1994, Opdam 1990) and some insects and some mammals are thought to occur as
metapopulations and survive because of regular dispersal to and re-colonisation of new and
vacated patches (English Nature 1993, Hanski et al. 1995, Lankester et al. 1991). However,
habitat fragmentation by roads is usually abrupt and often severe and there is frequently a
simultaneous reduction in habitat quality and population size. If new constructions fragment
an area in such a way as to leave habitat ‘islands' distant, disconnected and small then the
remaining populations may not be able to recover (Soule 1987). Roads can impose major
6
barriers to faunal movement, the intensity of the barrier being dependent on the intrinsic
nature of the highway and verge (Bennett 1991a, Bright 1993, English Nature 1993, 1996,
Mader 1984, Slater 1995, Vermeulen 1994). The effect of roads on specific mammals is well
documented (Bennett 1991a, Clarke et al. 1998, Huijser 1999, Korn 1991, Putman 1997
Richardson et al. 1997, Spellerberg 1998, Forman and Alexander 1998). Bennett (1991a)
summarised three major factors which influence the permeability of roads: the width of the
gap between suitable habitats (clearance), the relative mobility and behaviour of the animal,
and the contrast between the ‘barrier’ (the road surface and sometimes the verge as well) and
the adjacent habitat. The speed of the traffic, the size of the species and its dispersal
behaviour are also cited as important factors when assessing the barrier effect of a road (Van
Langevelde and Jaarsma 1995).
Wide roads with high traffic densities restrict animal
movement most severely. The largest and busiest roads are generally penetrated only by
dispersing individuals or when resources are scarce. Nevertheless, it is not just large roads or
busy roads which impede movement; gravel field-tracks can reduce the rate of crossing for
ground-foraging arthropods (Mader et al. 1990) and molluscs avoid pathways which lack
vegetation cover (Oggier 1997). All roads therefore can present some level of barrier and
increase landscape resistance but the influencing factors will vary greatly between species.
Whilst roads may restrict the directional movement of small animals, they constrain
movement rather than limit it absolutely.
In studies where small mammals have been
translocated to the opposite side of the road they frequently return to their home side (Korn
1991, Kozel and Fleharty 1979) and may do so at all times of the day and night irrespective of
the traffic volumes (Richardson et al. 1997) but such road crossings may merely indicate that
home ranges are confined to just one side of the road. Other studies clearly indicate that the
natural inclination of small animals is to avoid crossing roads, and to adopt roads as
7
boundaries to their normal home range. No naturally occurring road crossings by woodland
rodents (Apodemus flavicollis and Clethrionomys glareolus) were detected over a five year
period by Mader (1984) and road crossings of stenotopic carabid beetles were equally rare. In
another extensive trapping study of nearly 600 small (<500g) mammals Oxley et al (1974)
found that only 14 out of a total of 651 recaptured individuals (0.02%) crossed roads, and
roads which were wider than 30m were almost never crossed by small animals despite intertrap movements of over 200m.
Clearly, it is not uncommon for medium and large sized animals to cross all sizes of roads (as
evidenced by the high number of visible road casualties) but the indications are that like small
mammals, their movements are checked by large busy roads.
The frequency of road-
crossings by medium sized animals e.g. brown hare (Lepus europaeus), grey squirrel (Sciurus
carolinensis), and stoat (Mustela erminea) is greatly reduced with increasing road width
(Oxley et al. 1974); hedgehogs (Erinaceus europeus) generally avoid roads (Huijser 1999),
and badger (Meles meles) tend to avoid crossing wide roads with high traffic densities (Clarke
et al. 1998). All species of deer regularly cross minor roads but home-ranges are often
delimited by primary highways and only seasonal dispersal appears to provoke any frequency
of movement across larger, more heavily trafficked roads (Putman 1997).
The review of the literature shows that the severity and consequences of the barrier created by
roads varies. In an already fragmented landscape the barriers imposed by roads can seriously
curtail interactions between conspecific populations, and the limited gene flow which results
from this can render small populations vulnerable to inbreeding depression and to stochastic
events (Opdam 1990).
8
Roads as agents of mortality
Mortality rates for many species are often difficult to obtain. Some countries maintain a
national database for fauna casualties on roads but the records are generally for a limited
number of larger species, and the reliability of these and other estimates produced from
extrapolated data can often produce wide-ranging results. For example, annual estimates of
bird mortalities in the UK range from 30 million to 70 million (English Nature 1993). The
difficulties of accurate recording are not easy to resolve. Many animals which are seriously
injured will seek cover and die out of sight and, because of the speed at which corpses of
small animals are scavenged and disappear from the road or are crushed and destroyed by
passing vehicles, a single daily corpse census can seriously underestimate the death rate of
small animals. On a road where 179 toad corpses were counted at dawn all had disappeared
by 08.30 hrs; on average, a corpse remained for less than one hour during the daytime (Slater
1995).
Statistics for the number of road-kills in England and the UK are given in various reviews on
wildlife and roads (English Nature 1993, English Nature 1996, Slater 1995). Roughly one
million wild animals are thought to be killed on roads in the UK each year, including an
estimated 29-40% of all amphibians; 5000
barn owls (Tyto alba), or 30-60% of UK
population; 50,000 badger (Meles meles), or 49% of UK population (Clarke et al. 1998); 50100,000 hedgehog (Erinaceus europaeus), or up to 5% of UK population (Morris 1994) and
58% of the UK fox (Vulpes vulpes) population (Harris and White 1994). In the New Forest,
Hampshire more than 60 deer are reported killed each year and on Cannock Chase,
Staffordshire 180 are reported killed annually (English Nature 1996). Unlike mainland
Europe there are no British mammals which migrate large distances as part of a seasonal
pattern of activity.
These figures therefore relate to individuals killed on roads which
9
intersect their normal home territory, or killed crossing roads when dispersing from their natal
territory or when roaming during the breeding season.
In contrast to the highly intensified landscape of Britain, continental land masses retain large
tracts of continuous high forest and undeveloped areas which support a greater diversity and
abundance of animals, and more research effort is consequently focussed on the larger
mammal species. Many of these larger species have extensive home ranges and also follow
seasonal migratory routes which necessitate crossing many major highways, increasing their
exposure and vulnerability to road traffic. In Slovenia, where a stable population of 320-400
grizzly bears (Ursus arctos) occupy a range of 5000 km2 there were 10 reported road deaths
in a two year period (Kobler and Adamic 1999) whilst in Yellowstone National Park (an area
of 8,992 km2), there were 8 black bears (Ursus americanus) and 2 grizzly bears killed on the
roads in a 10 year period (Gunther and Biel 1999). In a Minnesota study, 11% of all known
wolf (Canis lupus) mortalities were caused by vehicle collisions (Paquet and Callaghan 1996)
and, also in the US, there were an estimated 538,000 deer killed on the road in 1991/2. In
Sweden 55,000 deer were killed on the road in 1996 and 12,000 were killed by vehicles in
Germany in the same year (Putman 1997). Thus on an international scale, roads and traffic
are a major cause of death to larger mammals. International interest in these incidents is
increased by the animal welfare issue when large animals are struck by cars, and also the
frequency of accidents means that the safety of motorists becomes a major consideration.
Factors which increase the risk of faunal road fatalities
Animals with high density in adjacent roadside verges, or which have large home ranges, or
which disperse widely are the most frequent traffic victims (Adams and Geis 1983, van
10
Langevelde and Jaarsma 1995). Medium and large sized mammals are particularly at risk,
especially when the emergence of young coincides with high traffic volumes (Oxley et al.
1974). Various species show seasonal peaks in accident rates often with a higher percentage
of males being killed (Davie et al. 1987, Mead 1997, Rotar and Adamic 1995, van
Langevelde and Jaarsma 1995). This suggests that breeding or dispersal behaviour may be
partly responsible but increases in summer time accidents may also be associated with higher
summer traffic levels (Moshe and Mayer 1998). Other species at risk of traffic accidents are
those which are attracted to or spend a disproportionate amount of time on a road such as
snakes which are attracted to the different microclimate of the road by its heat absorbing
surface (Spellerberg 1998), and large herbivores which are attracted by the minerals available
in rock salt deposited on roads to prevent freezing (Slater 1995). In the UK birds which use
roadside verges as a food resource, those which walk rather than fly across the road (such as
the moorhen, Gallinula chloropus), and corvids which scavenge on other road-kills are
particularly susceptible (Mead 1997).
Various factors contribute to the large number of road-related animal deaths (Clarke et al.
1998, Oxley et al. 1974, van Langevelde and Jaarsma 1995) but the predominant ones are
traffic density and road width. These two factors directly affect the success, or otherwise, of
an animal reaching the opposite side of the road with an increase in either reducing the
probability of the animal crossing safely. However very high traffic volumes can, perversely,
reduce some threats to wildlife. By suppressing activity in the vicinity of roads and limiting
the cross-over rate fewer animals are killed as a result of collisions with vehicles (Verboom
1995). A study of badgers undertaken by Clarke et al (1998) illustrates this effect. It revealed
that an increase in badger mortality was proportional to increases in traffic density but only up
to a certain traffic threshold above which badgers resisted crossing the road, and consequently
11
the proportional mortality rate fell.
Most accidents involving faunal casualties occur at night, coinciding with an increase in
activity for many species and a reduced field of vision for motorists. On English roads the
total animal death toll appears to be greater than that in other European countries (English
Nature 1993). This may be because English roads are not as straight as those elsewhere, or
because many English roads are hedge-lined, or it may be a combination of these or other
factors. Generally, the number of deaths is related to and influenced by the local landscape
although even the day of the week can be related to the numbers killed. Davie et al. (1987)
found that red fox deaths were highest on a Friday or Saturday night when the volume of
traffic is also generally higher.
Ecological Impact of Road Fatalities
If road mortalities are high they can impact at the population level. The decline of occupied
badger setts by some 30% in the Netherlands during a 20 year period from 1960-1980 is
attributed to traffic mortality (van der Zee et al. 1992). Similarly, although in the UK the
badger population seems currently able to withstand the loss from road casualties, Clarke et al
(1998) believed that if UK traffic volumes rise in line with the Department of Transport
predictions the loss of badgers from road mortality in combination with the high level of
habitat fragmentation in the UK may lead to future population declines.
In a sample
population of hedgehogs in the Netherlands, 2% were killed by traffic, and Huijser and
Bergers (1995) concluded that an effect at the population level could not be ruled out. Frog
and toad populations can be decimated by even fairly low volumes of traffic (Reh and Seitz
1990), and Fahrig et al (1995) suggest that toad populations could be in a state of global
12
decline as a result of the increase in traffic world-wide.
Anecdotal evidence from
questionnaires distributed to voluntary toad patrol groups in the UK identified traffic increase
as the factor considered to be most important in a perceived decline in toad populations
(Foster 1996).
Hard information is still lacking for the effect of roads and traffic at the population level
(Bennett 1991a). From the available evidence, the population effect appears generally to be
at a local level where there are small populations, or for endangered species (Bright 1993).
For most species road-kill does not seem to have a significant effect at the population level
(Reijnen and Foppen 1997). Munguira and Thomas (1992) found no apparent effect on the
populations of butterflies and Putman (1997) reports that the high accident rate of deer and
other ungulates is not sufficient to threaten population status. Nevertheless, the mortality rate
combined with the barrier effect of roads may become of increasing significance in a patchy
and fragmented landscape where local populations are increasingly reliant upon
metapopulation function and occasional dispersal of individuals from separated populations.
Road verges as habitat
In ecological terms roadside verges can be classified as edge habitat having extreme length
but very little depth. Edge habitat can provide for both the species typical of adjacent habitat
types and the specialised species of overlapping habitats (Way 1977). In the UK road verges
are frequently separated from the adjacent landscape by hedges and ditches and are often
managed differently from the surrounding landscape so they can feature remnant habitat
patches and different communities, including plant species that are poorly represented in the
locality. The loss of natural and semi-natural habitat has been so severe in the UK this
century (English Nature 1993) that roadside verges are indeed becoming increasingly
13
important as an extensive and relatively undisturbed habitat. In the last comprehensive
roadside survey in the U.K., Way (1977) recorded 870 of the 2000 different UK plant
species, 20 of the 50 species of mammal, 40 of the 200 species of bird, 25 of the 60 species of
butterfly, 8 of the 25 species of bumblebee, all 6 reptile species and 5 out of 6 species of
British amphibians. Road verges currently comprise almost 1% of the land surface of the
UK. It has been estimated that 10% of the land surface of Great Britain needs to be protected
in order to ensure that the majority of species will survive (National Conservancy Council
1984) but protection in the form of designated Sites of Special Scientific Interest (SSSIs) only
covers some 7% of the land surface at present. Road verges therefore have considerable
potential as an ecological resource and are likely to become increasingly important as refuges
for wildlife in intensified landscapes. Several roadside areas have already been designated for
their distinctive contribution to nature conservation, six as SSSI's, two as Specially Protected
Areas (a pan European designation to protect habitats of important species) and one as a
National Nature Reserve.
The fauna of road verges in the U.K. is diverse but the habitat is not suitable for all native
species. Invertebrates are generally plentiful on roadside verges. In agricultural landscapes,
verges provided a periodic refuge for retreating individuals escaping from agricultural
treatments in neighbouring fields (Mader 1984). On verges adjacent to heathland, Eversham
and Telfer (1994) observed several rare species of beetle which were more numerous on the
verge than on a nearby nature reserve. In other situations however where the road verge is
markedly different form the adjacent habitat (as in the case of adjacent woodland) 'interior'
species may avoid penetrating the verges altogether (Mader 1984) and, for some carabid
beetles the roadside can act as a sink habitat (Pulliam 1988) with populations maintained only
by continuous immigration (Vermuelen 1994). Road verges and the central reservations can
14
support a wide variety of butterflies including rare species. On road verges in Hampshire and
Dorset, 27 species of butterfly were recorded, representing 47% of butterflies species found in
the UK. The range of suitable breeding habitat, the width of the verge and the abundance of
nectar were factors which positively influenced the diversity and abundance of species, whilst
the volume of passing traffic is apparently no deterrent to breeding moth and butterfly species
(Munguira and Thomas 1992).
Birds may be attracted to road verges for foraging or
occasionally for breeding especially when the surrounding landscape is unsuitable for these
purposes. Eighteen different species of birds were recorded as using various sections of the
roadside verge in one Danish study (Laursen 1981). Skylarks (Alauda arvensis) were the
most abundant species and were found to forage more frequently on the road verge than in
adjacent fields. They were also found to favour the roadside as a nesting site when adjacent
fields provided inadequate cover early in the nesting season. Where open fields were the
predominant landscape cover, passerines such as the greenfinch (Carduelis chloris) and
starling (Sturnus vulgaris) were observed to travel long distances to feed on road verges.
However, on busy roads the noise levels had a negative effect on bird densities and it is
possible that birds only breed on the sub-optimal road verge habitat because of over-capacity
or lack of more suitable habitat rather than because it is a preferred nest site. Further
population level research is needed to determine the role of the verges in population dynamics
of these species. The undisturbed roadside areas also provides habitat for large numbers of
small mammals and birds, especially for edge and generalist species (Forman 1995) with a
corresponding increase in the number of predator species (Dawson 1994).
Road verges as movement facilitators
Paradoxically, whilst roads may be the source of much habitat fragmentation they may also be
a mechanism by which to restore connectivity in an intensive landscape. Due to their linear
15
nature, roads and their verges frequently cross environmental and topographical contours
(unlike 'natural' corridors) and can link a range of different habitats thus facilitating biotic
movement through an otherwise unsuitable landscape.
As a means of retaining and/or
enhancing connectivity, corridors, which connect habitat patches and so facilitate the
movement and dispersal of animals have been widely promoted (Beier and Noss 1998, Harris
and Scheck 1991, Loney and Hobbs 1991, Merriam 1991, Saunders and Hobbs 1991). It is
argued that corridors can enable colonisation and re-colonisation and thus prevent local
extinctions from accumulating into more widespread and irreversible extinctions.
Getz et al (1978) were able to show that voles (Microtus pennysylvanicus) extended their
range by some 90 km through utilisation of the verge of an interstate highway and the roads
connected to it, implying a corridor function for some species in certain conditions. Several
recent works have shown the use of roadside verges as habitat (Downes et al. 1997, Eversham
and Telfer 1994, Vermeulen 1994) and
Nicholls and Margules (1991) contend that if
corridors provide habitat which can maintain populations then it is possible to infer that they
may also provide a dispersal corridor which would permit re-colonisation following patch
extinctions.
Nevertheless, the 'corridor' theory is not without controversy and its critics assert that
corridors are limited in their application, that there is no evidence to show that species cannot
do without them and that there is a lack of empirical data in support of the theory (Bonner
1995, Dawson 1994, Rich 1994, Simberloffe et al 1992). Furthermore, there is a risk attached
to corridors insofar as if they fail to provide a throughway to favourable habitat at a reachable
distance they may function as a sink habitat whilst at the same time depleting the source
population (Pulliam 1988, Saunders and Hobbs 1991, Vermeulen 1994). There is also the
16
risk of invasive species or disease moving along corridors to areas that would not otherwise
be affected (Hess, 1994).
Traffic assisted dispersal
Vehicles can act as a non-selective means of conveyance, capable of carrying and dispersing
organisms over considerable distances.
Vehicle tyres and mud flaps are capable of
transporting the seeds and propagules of all roadside plant species (Lonsdale and Lane 1994,
Schmidt 1989), particularly small, fast growing species (Hodkinson and Thompson 1997)
with dispersal being greatest in the direction of traffic flow and with small seeds being carried
the furthest (Scott and Davidson 1985). Although largely ignored in the literature, there is
also the potential for assisted transport of fauna by vehicles (Bennett, 1991a). Invertebrates
frequently enter vehicles and may be transported long distances and, if there are unconfirmed
reported incidents in the UK of domestic cats occasionally travelling long distances
inadvertently after climbing into the undersides of cars, it is not unlikely that small animals
such as rodents may also hitch an unintended lift on occasions.
Management Considerations
Improving the nature conservation value of roadside verges
Responsibility for roadside management in the UK is divided between the Highways Agency
(motorways and all purpose trunk roads) and the various local highway authorities (all other
roads). The motorway network includes in its ‘green estate’ 18,000ha of grassland which,
until 1975, was sown with a standard high productive grass seed mix and mown regularly.
Current policy for the management and maintenance of all primary highways reflects a
17
combination of safety and fiscal measures tempered latterly with the recognition of the
potential contribution of road verges to the natural wildlife resource. The present policy for
motorways and most other main roads is to plant native species on verges and to cut only
sight lines and access areas. Between 1962 and 1990, 33 million trees plus a further 2 million
each year have been planted on the motorway ‘green estate’ in the UK.
In many areas the change to less intensive verge management has produced a closed rank
sward with a deep thatch, resulting in an abundance of coarse grasses and umbellifers with no
opportunities for sward diversity, especially where fertility is high (Parr and Way 1988,
Sangwine 1990). The Countryside Survey of 1990 (Barr et al. 1990) shows that overgrown
eutrophic verges which feature umbellifers and stinging nettles (Urtica dioica) are now the
most common type of roadside vegetation. However, the reduction in roadside verge cutting
which has been largely responsible for this, means that there is a concomitant reduction in the
level of disturbance and this favours small mammal species as well as giving significant
management cost savings. In contrast to the frequently observed mediocre, eutrophic verges,
there are some notable plant communities (as denoted by their status as SSSIs) flourishing at
the roadside, as well as a number of uncommon and red data book species such as thistle
broomrape, (Orobanche reticulata) and Watling street thistle (Erynigium campestre).
Barnsley Metropolitan Borough Council (1995) reports that the undisturbed grass verge
supplies nesting opportunities for the declining grey partridge (Perdix perdix) in an otherwise
limited surrounding habitat due to farming intensification, and a refuge for slow worms
(Anguis fragilis), badger (Meles meles), and adder (Vipera berus).
In the Netherlands, where the intensity of the road network equals that of the UK, road verge
management policy has also changed, largely in response to ambitious national targets for
18
nature conservation and de-fragmentation. Management is designed to be sensitive to nature
conservation and, as a result, endangered plant species of the Netherlands such as wild
parsnip (Pastinaca sativa) oxeye daisy (Leucanthemum vulgare) and sheep's bit (Jasione
montana) are thriving on the verges, as are sand lizards (Lacerta agelis) and viviparous
lizards (Lacerta viviparta) (Bekker et al. 1995).
Roadside linear habitats not infrequently accommodate faunal and floral species of
conservation interest but uncommon species require uncommon habitats and they therefore
may not thrive in habitats managed for species-richness. To optimise the nature conservation
value of roadside habitats different management strategies appropriate to the ecological needs
of specific target species are required. Inevitably, habitat that may be suitable for one species
may be inimical to another. It is necessary therefore to recognise the importance of the
management and treatments of the roadside verge and to make early decisions about the
desired outcome for the area.
Buffer zones
Buffer zones can be used to prevent degradation of core habitat and to reduce the undesirable
effects of edge (Angold 1997a). They can be established beside roads by increasing the width
of the road verge and softening the transition from adjacent habitats by planting or by natural
regeneration. Broadening the road verge will provide a margin between the road and any
adjacent core habitat and at the same time may assist linear movement along the verge,
providing habitat or refugia. Road verges often have high levels of species diversity but this
may be at the expense of other, arguably more desirable features. Wider road verges clearly
increase the available habitat and thus encourage a greater abundance of species which favour
19
that particular habitat type, as well as providing a buffer zone between core habitat and roadassociated pollutants. However, although rare species are found on roadside verges it is
invariably the common species which contribute most to the roadside composition. An
increase in the road verge area which results in a greater loss of the original habitat and its
associated flora and fauna should always be avoided. It should also be taken into account that
where broad road verges are responsible for an increase in faunal abundance there may be a
consequential increase in mortality rates from roadside accidents.
Improving the safety and permeability of roads
Allowing a severed habitat to extend to the verge on each side of the road will reduce the
clearance between favourable habitats and facilitate crossing whether or not mitigation
measures such as bridges, tunnels or culverts, designed or adapted for wildlife use are
employed. The conflict arising from this approach is that an increase in cross-over and a
reduction in sight lines along the perimeter of the road can both contribute to increases in
mortality. If the barrier effect of roads is to be reduced then both an increase in the safety and
an increase in the permeability of roads needs to be considered. Reduction in traffic volume
and speed in conjunction with a reduction in the width of the road will both substantially
contribute to ‘defragmentation’ of habitat and increase ecological safety. On minor roads, a
concentration of traffic on a limited number of roads is considered preferable from an
ecological viewpoint to diffusing traffic across the network (van Langevelde and Jaarsma
1995). Greater permeability of the road has been achieved in many European countries in
recent years through the provision of ‘eco-passages’ (a generic term for artificially
constructed underground or over-ground passageways designed to facilitate faunal movement
across roads). In the Netherlands more than 60 constructions have been introduced on new
roads in the last 10 years (Bekker et al. 1995) and in the UK, a 40 mile stretch of the newest
20
motorway, the M40, features 14 badger tunnels (Hepinstall and Blood 1993).
The monitoring of eco-passages has shown that they are used by many different animals
(Bekker et al. 1995) although their overall effectiveness in terms of reducing mortality and
promoting interaction between sub and meta-populatons is still being studied. The extent to
which location affects their use by different species, and the behaviour of animals when
confronted by passageways (which will determine whether or not they accept and use them)
also requires further investigation (Nieuwenhuizen and van Apeldoorn 1995). It is generally
agreed that to be effective wildlife passages should be designed with particular species in
mind and meet specified criteria; tunnel dimensions, for example, can greatly affect usage
(Janssen et al. 1995). However, even with good design not all animals will use the smaller
passages especially when the underground passages have to traverse long distances as, for
instance, under motorways. The alternatives are to construct viaducts, to tunnel the road or
construct ‘green’ bridges. Green bridges or eco-ducts have been used in many European
countries but the cost of installation either during initial construction, or of retrofit following
construction can be prohibitive. There are few UK examples - the bridge across the M25
connecting Epping Forest on the outskirts of London is an exception.
There is now widespread use of roadside fencing to prevent animals from wandering onto the
roads and, when properly erected and maintained fences are successful in reducing animal
mortality (Rotar and Adamic 1995). The greatest reductions in road casualties are realised
when fences are used to funnel animals towards a tunnel or eco-passage entrance and prevent
crossings elsewhere. The drawback of fencing is that whilst preventing mortalities it can
virtually eliminate crossover requiring an extensive network of eco-passages if habitat
connectivity is to be maintained. In Austria the fenced road network is almost total and
21
effectively divides the country into 14 habitat fragments; 543 eco-passages are presently
installed to improve the permeability of the fenced road network (Volk and Glitzner 1998).
Environmental Impact Assessment and Mitigation
The road building programme in the UK continues but at a rate and scale greatly reduced to
that envisaged in the early 1990’s. Nevertheless, it is likely that new roads will continue to
have a consequence in environmental terms.
Environmental impact assessment (EIA),
established by statute in the UK in July 1988 for all major road projects (there is no
equivalent for existing stuctures), is now a well established procedure and takes account of
factors which may prove damaging to wildlife and the natural surroundings. The EIA process
is required to critically examine proposals for new developments and to recommend measures
to avoid or ameliorate any adverse impacts arising from the proposed scheme but the
effectiveness of the procedure is considered by some, to be less than satisfactory. Direct
habitat loss is quantifiable and easily considered by the assessment process but issues such as
fragmentation, the barrier effect, wildlife mortality and the provision of wildlife corridors are
more controversial and the EIA procedure (allegedly) not only fails to be comprehensive in its
account of impacts but, the response to fragmentation is perceived as being determined often
by cost rather than appropriateness (Kirby 1997).
The lack of routine testing of the
predictions made in Environmental Assessments and the absence of long-term monitoring
and after-care procedures for areas affected by, or established as a result of, construction is
also considered to be disappointing (Cibien and Magnac 1998, Janssen et al. 1995, Marshall
et al. 1995, Therivel and Thompson 1996).
Guidance on the most appropriate methods to adopt in terms of mitigation for roads is not
straight-forward because at present, evidence to support different approaches is incomplete.
22
Also, many of the impacts will be habitat or species-specific, and therefore there can be no
all-purpose answers applicable to every situation. However whilst awaiting further evidence
and scientific knowledge, some general principles do apply. English Nature (1993) suggests
that comprehensive mitigation should pay attention to:
•
avoidance of the best and non-recreatable sites;
•
provision of acceptable mitigation, and preferably compensation, for all other habitats
affected;
•
scheme enhancements by habitat creation on roadsides or elsewhere, re-connection of
severed patches and improvement of poor quality habitat links;
•
a long term commitment to manage and maintain the mitigation package. Long-term
management and monitoring, and attention to the roadside habitat and its management, is
an essential aspect of highway maintenance.
CONCLUSIONS
New roads will inevitably lead to habitat loss and fragmentation. The ecological impacts will
depend on the nature and extent of the existing road network, and the degree to which natural
and semi-natural habitats are already fragmented and isolated by intervening land use. The
case study has shown that in the intensively used landscape of the UK, roads affect almost
half the designated sites of special scientific interest for nature conservation. Even when
roads do not directly destroy habitat the noise and disturbance associated with them may
impact significantly on those species which require an undisturbed and/or interior habitat. In
the case study it was shown that up to 25% of protected areas may be disturbed by their
proximity to a road and that habitat loss beneath roads and habitat alteration near the road are
proportionately greater where habitat fragments are smaller, as in an intensively used
23
landscape.
Roads will act as barriers to movement or agents of mortality for some species. With habitat
fragmentation movement of individuals through the landscape becomes increasingly
important if populations in small habitat patches are not to become increasingly isolated, and
therefore vulnerable to genetic change and stochastic events which may jeopardise their long
term persistence. In a highly modified landscape the evidence suggests that some species
respond by becoming increasingly sedentary so that isolation by habitat fragmentation is
intensified by genetic and behavioural modifications of the species. Conversely, if faunal
movement continues despite ever increasing traffic densities and without further provision for
safe passage, traffic fatalities will be an inevitable consequence which may in itself depress
populations of certain species. Roads, and the phenomenal increase in traffic levels in recent
years, are a relatively new evolutionary pressure and the effects of this new selective pressure
have yet to be fully understood.
There are still many gaps in our current knowledge about the processes which operate within
a heterogeneous and fragmented landscape. In the context of roads, research has so far
largely considered the effect of roads upon individuals and species (Table 3), this leaves
major questions over our understanding of the impacts of roads on biodiversity at the
population and community level.
These questions can only be addressed by long-term
population studies which will enable an understanding of population dynamics in relation to
the effects of roads and the efficiency of mitigation and compensation measures. Areas to
which further research needs to be directed include: the impacts of fragmentation on
populations, the value and effectiveness of corridors, the value and effectiveness of ecopassages at population level, the implications of traffic related mortalities at a population
24
level and an accumulation of data on mortalityies. General autecological and synecological
work needs to consider patch dimensions and configuration, the effect of disturbance,
dispersal behaviour and an assessment of remedial measures designed to offset the adverse
effects arising from both existing and future road construction.
In the absence of this research, it is sensible to apply the precautionary principle to avoid
irreversible losses of biodiversity arising from irreparable habitat fragmentation. Alongside
this it will be necessary to maintain the existing pattern and connectivity of the remaining
natural and semi-natural habitats. Possible natural corridors and migration routes should be
protected, and the creation of new or artificial barriers avoided. Engineering improvements
or alterations to existing roads can be used as an opportunity for environmental audits and for
ecological improvements through the inclusion of further mitigation, compensation and/or
enhancement measures, at minimal cost. A methodology to audit the ecological aspects of
existing roads is needed in order that we might sustain the nature conservation values in and
around them, and identify potential for ecological enhancement and where existing habitat is
fragmented by a road, measures created to improve connectivity and reduce landscape
resistance are required.
If the damaging effects of the road network are to be moderated an informed dialogue with
planners and developers based on a thorough understanding of the long term effects of roads
on wildlife is required. Without empirical evidence and its communication arguments for a
change in approach and practice will be unconvincing and financial stringency will invariably
dictate acceptance of the least-cost method. This will have inevitable adverse consequences,
and the process of decline, already apparent for many species may be exacerbated and
additional species put at risk. With the correct strategies in place adverse impacts can be
25
minimised and the ecological potential realised.
26
Acknowledgements
We would like to thank the Highways Agency, (an agency of the U.K. Government), for
funding this work as part of a three year research project, being undertaken by Jackie
Underhill, which is investigating the impacts of linear transport infrastructures on wildlife
and its habitats. No part of this article can in any way be construed as representing the views
of the Highways Agency.
27
REFERENCES
Adams, L.W. and Geis, A.D. 1983. Effects of roads on small mammals. Journal of Applied
Ecology 20:403-415.
Andren H. 1994. Effects of habitat fragmentation on birds and mammals in landscapes with
different proportions of suitable habitat: a review. Oikos 71:355-366.
Andrews, A. 1990. Fragmentation of habitat by roads and utility corridors: a review. Australian
Zoologist 26:130-141.
Angold, P.G. 1997a. Edge effects in fragmented habitats: the implication for nature
conservation. Pages 32-39 in K. Canters, editor. Habitat fragmentation and infrastructure.
Proceedings of the international conference on habitat fragmentation, infrastructure and the role
of ecological engineering, MECC, Maastricht, 18-21 September 1995. Ministry of Transport,
Public Works and Water Management, Delft, the Netherlands.
Angold, P.G. 1997b. The impact of roads upon heathland vegetation:the effect on plant species
composition. Journal of Ecology 34:409-417.
Barnsley Metropolitan Borough Council, editor. 1996. M1 junctions 35 - 38. Landscape
ecological appraisal. Highways Agency, London.
Barr, C.J., Bunce, R.G.H., Clarke, R.T., Fuller, R.M., Furse, M.T., Gillespie, M.K., Groom,
G.B., Hallam, C.J., Hornung, M., Howard, D.C. and Ness, M.J. 1990. Countryside Survey 1990.
28
Main Report. A Report prepared for the DoE by Institute of Terrestrial Ecology and the Institute
of Freshwater Ecology, components of the Natural Environment Research Council. Volume 2.
Department of the Environment, London
Baur, A. and Baur, B. 1990. Are roads barriers to dispersal in the land snail Arianta
arbustorum? Canadian Journal of Zoology 68:613-617.
Beier, P. and Noss, R.F. 1998. Do habitat corridors provide connectivity? Conservation
Biology 12(6): 1241-1252.
Bekker, H., Hengel, V.D.B. and van der Sluijs, H. editors. 1995. Natuur over wegen. Nature
over motorways. Ministry of Transport, Public Works and Water Management, Delft,
Netherlands.
Bennett, A.F. 1991a. Roads, roadsides and wildlife conservation: a review. Pages 99-118 in
D.A. Saunders and R.J. Hobbs, editors. Nature conservation 2: the role of corridors. Surrey
Beatty & Sons.
Bennett, A.F. 1991b. What types of organism will use corridors? Pages 407-408 in D.A.
Saunders and R.J. Hobbs, editors. Nature conservation 2. The role of corridors. Surrey Beatty,
Chipping Norton,NSW.
Bonner, J. 1995. Wildlife's road to nowhere. New Scientist 143:30.
Bright, P.W. 1993. Habitat fragmentation - problems and predictions for British Mammals.
29
Mammal Review 23(3/4): 101-111.
Canters, K.J. and Cuperus, R. 1997. Fragmentation of bird and mammal habitats by roads and
traffic in transport regions. in A. Coope and J. Power editors. Species dispersal and land use
processes. Proceeding of the sixth annual IALE (UK) conference, held at the Unversity of
Ulster, Colraine 9th-11th September 1997. IALE (UK) Aberdeen, Scotland, UK.
Cibien, C., and Magnac, M.-P. 1998. Etudes, recherches et mesures en faveur de la faune sur le
reseau routier: etat des lieux et propositions. in Paper presented at the 3rd conference, route et
faune sauvage., Strassbourg.
Clarke, P.G., White, P.C. and Harris, S. 1998. Effects of roads of badger Meles meles
populations in south-west England. Biological Conservation 86:117-124.
Davidson, A. W. 1971. The effects of de-icing salt on roadside verges. Journal of Applied
Ecology 8:555-561.
Davie, J.M., Roper, T.J. and Shepherdson, D.J. 1987. Seasonal distribution of road kills in the
European badger (Meles meles). Journal of Zoology 211:525-529.
Dawson, D., editor. 1994. Are habitat corridors conduits for animals and plants in a fragmented
landscape. English Nature, Peterborough. (English Nature Report no. 94)
DETR 1998. Transport Statistics for Great Britain 1998. HMSO, London.
30
Detwyler, T. R. 1971. Man’s impact on the environment. McGraw Hill, London.
Downes, S.J., Handasyde, A. and Elgar, M.A. 1997. The use of corridors by mammals in
fragmented Australian eucalypt forests. Conservation Biology 11(3,3 June):718-726.
English Nature. 1993. Roads and nature conservation. Guidance on impacts, mitigation and
enhancement. English Nature, Peterborough, Denise Ramsay (ed).
English Nature. 1996. The significance of secondary effects from roads and road transport on
nature conservation. English Nature Research Report 178. English Nature, Peterborough.
Eversham, B.C., and Telfer, M.G. 1994. Conservation value of roadside verges for stenotopic
heathland Carabidae: corridors of refugia. Biodiversity and Conservation 3:538-545.
Evink, G., Garret, P., Zeigler, D. and Berry, J.
editors.
1996.
Trends in addressing
transportation related wildlife mortality. Publication FL-ER-58-96, State of Florida Department
of Transportation, Tallahassee, Florida.
Evink, G., Garret, P., Zeigler, D. and Berry, J. editors. 1998. Proceedings of the international
conference on wildlife ecology and transportation. Publication FL-ER-69-98. State of Florida
Department of Transportation, Tallahassee, Florida.
Evink, G., Garret, P. and Zeigler, D. editors. 1999. Proceedings of the third international
conference on wildlife ecology and transportation. Publication FL-ER-73-99. State of Florida
Department of Transportation, Tallahassee, Florida.
31
Fahrig, L., and Merriam, G. 1994. Conservation of fragmented populations. Conservation
Biology 8(1):50-59.
Fahrig, L., Pedlar, J.H., Pope, S.E., Taylor, P.D. and Wegner, J.F. 1995. Effect of road traffic
on amphibian density. Biological Conservation 73:177-182.
Farmer, A. M. 1993. The effects of dust on vegetation – a review. Environmental Pollution
79:63-75.
Fehlberg, U. 1994. Ecological barrier effects of motorways on mammalian wildlife – an animal
protection problem. Deustch Tierarztliche Wochenschrift 101:125-129.
Fluckiger, W., Oertli, J. J., and Fluckiger-Keller, H. 1978. The effect of wind gusts on leaf
growth and foliar water relations of aspen. Oecologia 34:101-106.
Forman, R.T.T. 1995. The ecological effects of roads and the ‘spatial solution’ for land
mosaics. in K. Canters, editor. Habitat fragmentation and infrastructure. Proceedings of the
international conference on habitat fragmentation, infrastructure and the role of ecological
engineering, MECC, Maastricht, 18-21 September 1995. Ministry of Transport, Public Works
and Water Management, Delft, the Netherlands.
Forman, R.T.T. and Alexander, L.E. 1998. Roads and their major ecological effects. Annual
review of Ecological Systems 29: 207-231.
32
Foster, J. 1996. Toad Patrols: a survey of voluntary effort involved in reducing road trafficrelated mortality in amphibians. Froglife Report No1.
Foster, A. C. & Maun, M. A. 1978. Effects of highway de-icing agents on Thuja occidentalis in
a greenhouse. Canadian Journal of Botany 56:2760-2766.
Getz, L.L., Cole, F.R. and Gates, D.L. 1978. Interstate roadside as dispersal routes for
Microtus pennysylvaticus. Journal of Mammalogy 59:208-212.
Gunther,K. and Biel, M, 1999. Reducing human caused black and grizzly bear mortality along
roadside in corridors in Yellowstone National Park in G.L. Evink, P. Garrett, and D. Zeigler
editors. Proceedings of the Third International Conference on Wildlife Ecology and
Transportation. FL-ER-73-99. Florida Department of Transportation, Tallhassee, Florida.
Hanski, I., Pakkala, T., Kuussaari, M. and Lei, G. 1995. Metapopulation persistence of an
endangered butterfly in a fragmented landscape. Okios 72:21-28.
Harris, L.D., and Scheck, J. 1991. From implication to applications: the dispersal corridor
principle applied to the conservation of biological diversity. Pages 189-220 in D.A. Saunders
and R.J. Hobbs, editors. Nature Conservation 2: the role of Corridors. Surrey Beatty & Sons.
Harris, S., and White, P., editors. 1994. The red fox. The mammal society, London.
Hepinstall, C.T.K., and Blood, J.D. 1993. M40 Oxford to Birmingham route location and
design. Pages 21-32 in Road Panel: Proceedings of the Institution of Civil Engineers and
33
Transport. Volume 100, Paper 10024.
Hess, G.R. 1994. Conservation corridors and contagious disease: a cautionary note. Biological
Conservation 8(1):256-262.
Hodkinson, D.J. and Thompson ,K. 1997. Plant dispersal: the role of man. Journal of Applied
Ecology 34:1484-1496.
Huijser, M.P. 1999. Human impact on populations of hedgehogs Erinaceous europaeus
through traffic and changes in the landscape: a review. Lutra 42: 39-57.
Huijser, M., and Bergers, P.J.M. 1995. The mortality rate in a hedgehog (Erinaceus europaeus)
population: the relative importance of road kills. in Habitat fragmentation and infrastructure.
Janssen, A.A.A.W., Lenders, H.J.R. and Leuven, R.S.E.W. 1995. Measures combating the
effect of habitat fragmentation on the badger (Meles meles) in the Netherlands: ecological
corridors or pitfalls? in K. Canters, editor. Habitat fragmentation and infrastructure. Proceedings
of the international conference on habitat fragmentation, infrastructure and the role of ecological
engineering, MECC, Maastricht, 18-21 September 1995. Ministry of Transport, Public Works
and Water Management, Delft, the Netherlands.
Jones, P. H. 1981. Snow and ice control and the transport environment. Environmental
Conservation 8:33-38.
34
Jones, J. A. and Grant, G. E. 1996. Peak flow responses to clearcutting and roads in small and
large basins, Western Cascades, Oregon. Water Resources Research 32:959-974.
Kirby, K.J. 1997. Habitat fragmentation and infrastructure: problems and research. in K. Canters
editors. Proceedings of the international conference on habitat fragmentation, infrastructure and
the role of ecological engineering, MECC, Maastricht, 18-21 September 1995. Ministry of
Transport, Public Works and Water Management.
Kobler, A. and Adamic, M.
1999.
Brown bears in Slovenia: identifying locations for
construction of wildlife bridges across highways. in: Evink, P. Garrett, and D. Zeigler editors.
Proceedings of the Third International Conference on Wildlife Ecology and Transportation. FLER-73-99. Florida Department of Transportation, Tallhassee, Florida.
Korn, H. 1991. Rapid re-population by small mammals of an area isolated by roads. Mammalia
55:629-632.
Kozel, R.L., and Fleharty, E.D. 1979. Movements of rodents across roads. The southwestern
naturalist 24:239-248.
Lankester, K. van Apeldoorn, R., Verboom, J. 1991. Management perspectives for populations
of the eurasian badger Meles meles in a fragmented landscape. Journal of Applied Ecology
28:570-573.
Laursen, K. 1981. Birds on roadside verges and the effect of mowing on frequency and
distribution. Biological Conservation 20:59-68.
35
Loney, B., and Hobbs, R.J. 1991. Management of vegetation corridors: maintenance,
rehabilitation and establishment. Pages 299-311 in D.A. Saunders and R.J. Hobbs, editors.
Nature Conservation 2: The role of corridors. Surrey, Beatty & Sons.
Lonsdale, W.M., and Lane, A.M. 1994. Tourist vehicles as vectors of weed seeds in Kakadu
National Park, northern Australia. Biological Conservation 69:277-283.
Mader, H.J. 1984. Animal habitat isolation by roads and agricultural fields. Biological
Conservation 29:81-96.
Mader, H. J., Schell, C. and Kornacker, P. 1990. Linear barriers to arthropod movements on the
landscape. Biological Conservation 54:209-222.
Maltby, L., Forrow, D. M., Boxall, A. B. A., Calow, P. and Betton, C. I.. 1995. The effects of
motorway runoff on freshwater ecosystems. Environmental Toxicology and Chemistry 14:10791092.
Marshall, C., Corner, A. and Tattersfield, P. 1995. An amphibian mitigation scheme for the
A34 Wilmslow and Handforth Bypass, Cheshire, United Kingdom. in K. Canters, editor.
Habitat fragmentation and infrastructure. Proceedings of the international conference on habitat
fragmentation, infrastructure and the role of ecological engineering, MECC, Maastricht, 18-21
September 1995. Ministry of Transport, Public Works and Water Management, Delft, the
Netherlands.
36
Mead, C. 1997. A pathetic bundle of feathers. British Wildlife 8(4): 229-231.
Merriam, G. 1991. Corridors and connectivity: animal population in heterogeneous
environments. Pages 133-142 in D.A. Saunders and R.J. Hobbs, editors. Nature Conservation 2:
the role of corridors. Surrey Beatty & Sons, Chipping Campden.
Morris, P., editor. 1994. The hedgehog. The mammal society, London.
Moshe, I., and Mayer, R. 1998. Spatio-temporal trends in armadillo diurnal activity and roadkills in central Florida. http.www.nal.usda.gov/ttic/tektran/data/000008/60/0000086075.html.:1.
Munguira, M.I., and Thomas, J.A. 1992. Use of road verges by butterfly and burnet populations
and the effect of roads on adult dispersal and mortality. Journal of Applied Ecology 29:316-329.
Muskett, C. J. and Jones, M. P. 1980. The dispersal of lead, cadmium and nickel from motor
vehicles and effects on roadside invertebrate macrofauna. Environmental Pollution 23:231-242.
National Conservancy Council, editor. 1984. Nature conservation in Great Britain. NCC,
Peterborough.
Nicholls, A.O., and Margules, C.R. 1991. The design of studies to demonstrate the biological
importance of corridors. Pages 49-61 in A. Saunders and R.J. Hobbs, editors. Nature
conservation 2. The role of corridors. Surrey Beatty and Sons, chipping Norton, New South
Wales.
37
Nieuwenhuizen, W., and van Apeldoorn, R.C., editors. 1995. Project Versnippering. Deel 20A
Mammal use of fauna passages on natinal road A1 at Oldenzaal. Ministry of Transport, Public
Works and Water Management, Delft, Netherlands.
Oggier, P. 1997. roads as barriers to the dispersal of the endangered land snail Helicella itala.
in K. Canters, editor. Habitat fragmentation and infrastructure. Proceedings of the international
conference on habitat fragmentation, infrastructure and the role of ecological engineering,
MECC, Maastricht, 18-21 September 1995. Ministry of Transport, Public Works and Water
Management, Delft, the Netherlands.
Opdam, P. 1990. Dispersal in fragmented populations: the key to survival. Pages 3-17 in R.G.H.
Bunce and D.C. Howard, editors. Species dispersal in agricultural habitats. Belhaven, London.
Oxley, D.J., Fenton, M.B. and Carmody, G.R. 1974. The effect of roads on populations of
small mammals. Journal of Ecology 11: 51-59.
Paquet, P. and Callaghan, C. 1996. Effects of linear developments on winter movements of
gray wolves in the Bow River valley of Banff National Park, Alberta. in G.L. Evink, P. Garrett,
D. Zeigler and J. Berry (editors). Proceedings of the Transportation Related Wildlife Mortality
Semina. FL-ER-58-96. Florida Department of Transportation, Tallhassee, Florida.
Parr, T.W., and Way, J.M. 1988. Management of roadside vegetation: the long term effects of
cutting. Journal of Applied Ecology 25:1073-1087.
Pulliam, H.R. 1988. Sources, sinks and population regulation. American Naturalist 132:65238
661.
Putman, R.J. 1997. Deer and road traffic accidents: options for management. Journal of
Environmental Management 51:43-57.
Reh, W., and Seits, A. 1990. The influence of land use on the genetic structure of population of
the common frog Rana temporaria. Biological Conservation 54:239-249.
Reijnen, R., and Foppen, R. 1997. disturbance by traffic of breeding birds: evaluation of the
effect and considerations in planning and managing road corridors. Biodiversity and
Conservation 6:567-581.
Reijnen, R., Foppen, R., ter Braak, C. and Thissen, J. 1995. The effects of car traffic on
breeding bird populations in woodland. Journal of Applied Ecology 32:187-202.
Rich, T.C.G. 1994. Wildlife Corridors. In Practice 11.
Richardson, J.H., Shore, R.F., Treweek, J.R. and Larkin, S.B.C. 1997. Are major roads a
barrier to small mammals? Journal of Zoology 243:840-846.
Rotar, J.T., and Adamic, M. 1995. Wildlife traffic relations in Slovenia. Problems arising with
the construction of motorway network and possibilities of their mitigation. in K. Canters, editor.
Habitat fragmentation and infrastructure. Proceedings of the international conference on habitat
fragmentation, infrastructure and the role of ecological engineering, MECC, Maastricht, 18-21
September 1995. Ministry of Transport, Public Works and Water Management, Delft, the
39
Netherlands.
Salim, M. 1989. Effects of salinity and relative humidity on growth and ionic relations of plants.
New Phytologist 113:13-20.
Sangwine, A. 1990. Roadside Horticulture. Landscape Design 190:38-40.
Sarkar, R. K., Banerjee, A. and Mukherji, S. 1986. Acceleration of peroxidase and catalase
activities in leaves of wild dicotyledonous plants, as an indication of automobile exhaust
pollution. Environmental Pollution (Series A) 42:289-295.
Saunders, D.A., and Hobbs, R.J. 1991. The role of corridors in conservation: what do we know
and where do we go? Pages 421-427 in D.A. Saunders and R.J. Hobbs, editors. Nature
conservation 2: the role of corridors. Surrey Beatty.
Schmidt, W. 1989. Plant dispersal by motor cars. Vegetatio 80:147-152.
Schonewald- Cox, C. and Buechner, M. 1992. Park protection and public roads. In: Fieldler, P.
L. and Jain, S. K. (eds). Conservation Biology – the theory and practice of nature conservation,
preservation and management. Chapman and Hall, London.
Scott, N. E. 1985. The updated distribution of maritime species on British roadsides. Watsonia
15:381-386.
Scott, N. E. and Davidson, A. W. 1982. De-icing salt and the invasion of road verges by
40
maritime plants. Watsonia 14:41-52.
Scott, N.E., and Davidson, A.W. 1985. The distribution and ecology of coastal species on
roadsides. Vegetatio 62:433-440.
Seabrook, W. A. and Dettmann, E. B. 1996. Roads as activity corridors for cane toads in
Australia. Journal of Wildlife Management 60:363-368.
Simberloff, D., Farr, J.A., Cox, J. and Mehlaman, D.W.
1992.
Movement Corridors:
Conservation bargains or poor investments? Conservation Biology 6(4 December):493-504.
Slater, F. 1995. Wildlife Road Casualties. British Wildlife 5:214-215.
Soule, M.E., editor. 1987. Viable populations for conservation. Cambridge University Press,
Cambridge.
Spellerberg, I.F. 1998. Ecological effect of roads and traffic: a literature review. Global ecology
and biogeographical letters 7:317-333.
Spellerberg, I. F. and Gaywood, M. J. 1993. Linear features: linear habitats and wildlife
corridors. English Nature Research Report No 60. 30pp. English Nature, Peterborough, UK.
Therivel, R., and Thompson, S. 1996. Strategic environmental assessment and nature
conservation. Report to English Nature. English Nature, Peterborough.
41
Thompson, J. R. and Rutter, A. J. 1986. The salinity of motorway soils. Journal of Applied
Ecology 23:299-315.
Thompson, J. R., Rutter, A. J. and Ridout, P. S. 1986. The salinity of motorway soils 2.
Distance from the carriageway and other local sources of variation in salinity. Journal of
Applied Ecology 23:269-280.
Tyser, R. W. and Worley, C. A. 1992. Alien flora in grasslands adjacent to road and trail
corridors in Glacier National Park, Montana USA. Conservation Biology 6:253-262.
van Apeldoorn, R.C. 1995. Fragmented mammals; what does that mean? in K. Canters, editor.
Habitat fragmentation and infrastructure. Proceedings of the international conference on habitat
fragmentation, infrastructure and the role of ecological engineering, MECC, Maastricht, 18-21
September 1995. Ministry of Transport, Public Works and Water Management, Delft, the
Netherlands.
van der Zande, A.N., Keurs, W.J. and van der Weijden, W.J. 1980. The impact of roads on the
densities of four bird species in an open field habitat - evidence of a long-distance effect.
Biological Conservation 18:299-321.
van der Zee, F.F., Weitz, J., ter Braak, C.J.F., van Apeldoorn, R.C. and Vink, J. 1992.
Landscape change as a possible cause of the badger Meles meles L. decline in the Netherland.
Biological Conservation 61:17-22.
van Langevelde, F., and Jaarsma, C.F. 1995. Habitat fragmentation, the role of minor rural roads
42
(MRRs) and their traversability. in K. Canters, editor. Habitat fragmentation and infrastructure.
Proceedings of the international conference on habitat fragmentation, infrastructure and the role
of ecological engineering, MECC, Maastricht, 18-21 September 1995. Ministry of Transport,
Public Works and Water Management, Delft, the Netherlands.
Verboom, J., editor. 1995. Project Versnippering 23A. Dispersal of Animals and Infrastructure.
A model study: summary. Ministry of Transport, Public Works and Water Management, Delft,
Netherlands.
Vermeulen, H.J. 1994. Corridor function of a road verge for dispersal of stenotopic heathland
ground beetles carabidae. Biological Conservation 69:339-349.
Volk, F., and Glitzner. 1998. Assessment of barrier effects on red deer due to motorways in
Austria. First steps at assessing the permeability of the 1990k fenced motorways. Poster
presentation at the 3rd conference, route et faune sauvage. (Strasbourg)
Way, J.M. 1977. Roadside verges and conservation in Britain: a review. Biological
Conservation 12:65-74.
Welch, D. & Welch, S. 1988. Occurrence of Armeria maritima (Mill.) Willd. on an inland
roadside in north eastern Scotland. Watsonia 17:96-97.
43
Table 1. A summary of the ecological impacts of extant roads upon local biota.
ECOLOGICAL IMPACT
POLLUTION:
Foreign material used in
construction.
Dust.
De-icing salt.
EFFECT
SOURCE
May cause local pH change.
Detwyler 1971.
Affecting photosynthesis, respiration,
transpiration and facilitating pollutant
impacts.
Causes local salination and the spread of
maritime species along verges.
Farmer 1993.
Exhaust output including
Effects include stunted plant growth,
carbon monoxide, sulphur
increased heavy metal concentration in biota,
dioxide, nitrogen oxides,
and changes in ecological community
ozone, organic gases (e.g.
composition.
ethylene) and heavy metals
(e.g. lead).
CHANGES IN LOCAL HYDROLOGY.:
Increased runoff from
Pollutants may enter the stream network and
impervious surfaces.
cause changes in the diversity and
Pollutants such as
composition of aquatic macroinvertebrates.
hydrocarbons and heavy
metals in surface run-off
from the road
Changes in streamflow.
Culverts can alter water tables in the vicinity,
and roadside ditches connected to the stream
network cause higher, earlier discharge and
greater erosion and sedimentation.
DISTURBANCE EFFECTS:
Gusts of wind from passing
May inhibit plant growth and cause necrosis
vehicles.
(yellowing) of leaves near roads.
Increased human access and disturbance cause reductions in bird
noise.
population densities near roads in the
Netherlands.
PHYSICAL BARRIERS TO THE MOVEMENT OF ANIMAL SPECIES:
Barrier effect.
Roads act as physical barriers to some
species, and hinder the dispersal of others.
Fauna mortality.
the amount of wildlife killed on roads is very
much greater than was once thought.
PROVISION OF ECOLOGICAL HABITAT AND CORRIDORS:
Provision of linear habitat
The ecological and conservation value of
on the road verge.
road verges has been demonstrated.
Provision of ecological
There is considerable interest in the theory
corridors along road verges. that road verges act as ecological corridors,
but so far little hard evidence to substantiate
the theory. Roadsides do however play a
prominent role in the establishment and
dispersal of the alien flora, and the
introduced cane toad in Australia uses roads
as activity and dispersal corridors.
Davidson 1971, Foster and Maun 1978,
Jones 1981, Salim 1989, Scott 1985, Scott
and Davidson 1982, Thompson and Rutter
1986, Thompson et al. 1986, Welch and
Welch 1988.
Schonewald-Cox and Buechner 1992, Sarkar
et al. 1986, Muskett and Jones 1980, Angold
1997b.
Maltby et al. 1995.
Jones and Grant 1996.
Fluckiger et al. 1978.
Reijnen et al. 1995.
Andrews 1990, Baur and Baur 1990, Mader
1984, Mader et al 1990, Reh and Seits 1990.
Fehlberg 1994.
Way 1977.
Spellerberg and Gaywood 1993, Tyser and
Worley 1992, Seabrook and Dettmann 1996.
44
45
Table 2. The impact of roads on Sites of Special Scientific Interest (SSSIs). Data for
Oxfordshire UK.
Size of SSSI
Number (%) of sites
Combined area (%) of sites
Number (%) of sites with:
no road
a road adjacent
a road through
% habitat affected by roads through sites
(% of combined area) assuming:
5m road width
5m road width plus 5m impact zone
either side
5m road width plus 100m impact
zone either side
small
(<5ha)
53 (42%)
96 ha (2%)
medium
(5-50ha)
56 (44%)
1160 ha (28%)
large
(>50ha)
18 (14%)
2890 ha (70%)
Total
127
4146 ha
31 (58%)
19 (36%)
3 (6%)
35 (63%)
17 (30%)
4 (7%)
6 (33%)
5 (28%)
7 (39%)
72 (57%)
41 (32%)
14 (11%)
10% (1%)
30% (2%)
6% (1%)
17% (2%)
2% (1%)
6% (2%)
2% (1%)
7% (2%)
100% (2%)
100% (10%)
80% (28%)
90% (23%)
46
Table 3. The current extent of research on the different impacts of fragmentation resulting
from roads, indicating the level at which the research has been undertaken, the research gap
and the level at which research is still required to fill that gap.
Topic
Current Research Level
Research Gaps
Approach/Method
Barriers
individual / species
population /
multi-species & probably subcommunity level community approach; long term
population studies
Corridors
individual / species
population /
multi-species & probably subcommunity level community approach, long term
population studies
Mortality
individual / species
population
long-term population studies
/community
Habitat
population / community
population /
survey, long-term monitoring
community level
Eco-Passages individual / species
population /
multi-species approach, long term
community level population study
47